In electroerosive machining, the term herein used to refer to a machining method which involves the removal of material from a conductive workpiece at least in part by the action of electrical discharges, a tool electrode is spacedly juxtaposed with the workpiece across a machining gap flooded with a machining liquid and an erosion machining current, commonly in the form of a succession of electrical pulses, is passed between the tool electrode and the workpiece. Successive electrical discharges are thus produced across the liquid flooded machining gap to electroerosively remove material from the workpiece. As material removal proceeds, the total electrode supported by a tool head is advanced by a servo system to maintain the machining gap which tends to enlarge substantially constantly.
In the erosion process, the erosive wear of the machining face of the tool electrode may simultaneously occur. While various techniques such as "shaping" the erosive pulses individually or in groups, controlling the liquid delivery into the machining gap and the use of a particular electrode material have been found to be effective to minimize the erosive wear of the tool electrode or to achieve what is called the "no wear" mode, tool wear is nevertheless unavoidable in certain grades of machining operation such as finishing or microfinishing and should even be produced positively in such and other machining operations in the interest of achieving an increased machining efficiency. The particular kind of erosive machining which makes the "wear" mode operation unavoidable or even desirable makes use of a tool electrode which is typically slender and of simple cross section, e.g. a wire electrode or rod electrode of circular or square cross-sectional shape, for machining a large and/or intricate three-dimensional cavity in the workpiece. The cavity is then formed in the workpiece by controlledly displacing the tool electrode relative to the workpiece multi-axially in a three-dimensional coordinate system along a prescribed set of movement paths which determines the contour of the desired cavity.
In those machining operations which advantageously entail the "wear" mode, it will be apparent that it becomes critically important to precisely ascertain the wear of the electrode machining face on an instantaneous basis so as to allow the machining gap to be optimally maintained.
It has been recognized, however, that the rate of wear of the tool electrode depends upon a variety of machining factors which include the tool material and configuration, the selected parameters of the erosive pulses and the particular kind of the machining liquid and its particular manner of supply to the gap region. They also include the instantaneous machining depth and the configuration of machined portions as well as their physical interactions with the flows of the machining liquid which may vary instantaneously. In practice, therefore, such a large number of intricate factors cannot be correlated to precisely ascertain the rate of tool wear or to prepare an adequate program for a control system designed to advance the tool electrode in such a manner as to accurately compensate for the actual tool wear. Even an empirical program based upon an individual trial machining operation can hardly be obtained. The conventional approach taken to ascertain the tool wear is, therefore, on an in-process basis which monitors and analyzes the state of erosive discharges. For example, the ratio of the number of no-load pulses or arcing pulses to the total number of pulses applied per unit time, the discharge initiation voltage or the gap impedance has been measured and the electrode servo system arranged to controlledly advance the tool electrode so as to maintain any one of these variables at a given constant value.
The discharge state, however, does not exclusively represent the physical size of the machining gap but also reflect on the degree of contamination of the machining liquid, the machining pulse conditions and other gap variables generally and it has been found to be practically impossible to extract therefrom information exclusively relevant to the physical magnitude of the machining gap. Thus, a state in which satisfactory discharges continue does not always represent the machining gap as being of a selected size and it is altogether possible that a continuation of satisfactory discharges does not yield a desired machining accuracy.